1. Field of the Invention
The present invention relates generally to the field of battery powered electronic devices; and, more particularly, to a method and apparatus for calibrating a rechargeable battery for mobile telephones and other portable electronic devices.
2. Description of the Prior Art
Many types of electronic devices utilize battery power instead of or in addition to A/C power from an outlet. For example, mobile telephones and other mobile terminals, laptop computers, camcorders, etc., usually utilize rechargeable batteries; and a variety of types of rechargeable batteries have been developed for use in such applications including nickel-cadmium batteries, nickel metal hydride batteries and lithium ion batteries.
A user of a portable electronic device often wishes to know the operational time remaining with an installed battery so as to be able to properly plan for future usage of the device, and to generally know when the battery should be recharged. For example, a user of a mobile telephone may wish to know if expected calls can be made or received, that the phone can be properly used when taken on a long trip, etc.
Recognizing the importance of providing users of portable electronic devices with information concerning the remaining operational time of their devices, manufacturers often provide portable electronic devices, such as mobile telephones, with a xe2x80x9cfuel gaugexe2x80x9d capability by which the remaining capacity of a battery maybe monitored. In general, a fuel gauge calculates the remaining capacity of the battery as follows:
Remaining battery capacity=Total battery capacityxe2x88x92Used battery capacity 
A mobile telephone, for example, can measure the total current that is fed into it from the battery, and from this information can calculate how much capacity is withdrawn from the battery (i.e., used battery capacity). If it also knows the total recoverable battery capacity (usually referred to herein simply as the xe2x80x9ctotal battery capacityxe2x80x9d), it can easily calculate the remaining battery capacity using the above equation. Once the remaining battery capacity is known, the telephone can predict the remaining xe2x80x9ctalkxe2x80x9d and xe2x80x9cstandbyxe2x80x9d times for the telephone and provide this information to the user via a display or the like.
In order to avoid possible confusion, it should be noted that the term xe2x80x9cbatteryxe2x80x9d as used herein generally refers to a xe2x80x9cbattery packxe2x80x9d as is used to power many electronic devices. A battery pack typically includes one or a plurality of individual rechargeable battery cells. The individual battery cell is generally referred to herein as a xe2x80x9cbattery cellxe2x80x9d or, more simply, as a xe2x80x9ccellxe2x80x9d.
A value indicative of the total capacity of a cell is typically provided by the vendor of the cell, and a value indicative of the total capacity of the battery can be determined from this information. In the case of xe2x80x9cdumb batteriesxe2x80x9d, when the software for an electronic device is originally designed, the total battery capacity value is programmed into the device. With respect to xe2x80x9csmart batteriesxe2x80x9d, the total battery capacity value is provided in a memory storage device associated with the battery; and the device is able to communicate with the battery to obtain the total battery capacity value.
The total cell capacity value provided by the cell vendor, however, is a xe2x80x9cnominal valuexe2x80x9d, i.e., an average minimum capacity value of a fully charged cell for a particular cell type. Because it is an average minimum capacity value, it can differ significantly from the actual total capacity of individual cells of that particular type (a difference of as much as 20% has been observed); and, accordingly, the total capacity value of a battery incorporating the cell or cells can also differ significantly from the actual total capacity of that battery. This difference can have a very significant negative impact on the accuracy of the fuel gauging.
Recognizing the inadequacies of basing a determination of remaining battery capacity on the xe2x80x9cnominal valuexe2x80x9d provided by the vendor of the battery cell, it is known that the battery should be periodically calibrated in order to increase the accuracy of the fuel gauging. The calibration process, in effect, endeavors to accurately determine the actual total capacity of the battery at the specific time that the calibration is performed so that the calibrated value maybe used by the mobile phone or other electronic device to more accurately calculate the remaining battery capacity.
One well-known calibration procedure is based on the ageing of the battery. Specifically, it is well-known that the total capacity of a battery degrades as an effect of ageing; and to calibrate a battery using this procedure, the electronic device monitors the ageing process and adjusts the total battery capacity value as a function of battery ageing for use when calculating the remaining capacity of the battery.
Usually, the ageing of a battery is estimated by the electronic device based upon the number of completed charging cycles. Such an estimate, however, is only a rough estimate (a straight line estimate) and is not very accurate. In actuality, the effect of ageing is different for each individual battery and varies depending on a number of factors including temperature and usage intensity. In general, battery ageing is a very unpredictable process and current battery ageing estimations are not sufficient to provide an accurate measure of remaining battery capacity; and, hence, prevent an accurate estimation of remaining operational time of a device using the battery.
Various other procedures for calibrating a battery for a mobile telephone are known. These procedures are typically required to be initiated by the user of the telephone. In addition, if no dedicated hardware is added to the phone, the calibration procedure can take a very long time; and, since the phone normally cannot be used during the procedure, this can be a great inconvenience to the user. For example, in known calibration procedures, a fully charged battery is usually used. The battery is discharged completely while measuring the current and time; i.e., the capacity withdrawn from the battery, so as to provide a measure of the total capacity of the battery. Inasmuch, however, as mobile telephones may have a standby time of hundreds of hours, such a calibration procedure may require as much as several days.
The present invention provides a method and apparatus for calibrating a battery used, for example, to power a mobile terminal, such as a mobile telephone, or another electronic device.
A method for calibrating a rechargeable battery for an electronic device according to the present invention comprises the steps of charging the battery by a charging process, determining a total amount of charge accepted by the battery during the charging process, and providing a value of total battery capacity of the battery based, at least in part, on the total amount of charge accepted by the battery.
With the present invention, an accurate calibration of the battery can be carried out in a relatively short period of time during charging of the battery. For example, mobile telephones manufactured by the assignee of the present invention charge batteries using a fast charging algorithm, such that a complete charging cycle typically lasts no more than 1-2 hours. With the present invention, therefore, the battery can also be accurately calibrated in this period of time. In general, it is contemplated that a calibration according to the present invention will be carried out automatically during a typical overnight charging cycle so as to minimize possible inconvenience to the user.
According to a presently preferred embodiment of the invention, the step of determining a total amount of charge accepted by the battery comprises determining the net current fed into the battery during the charging process and the duration of the charging process; from which information, the total amount of charge accepted by the battery can be readily calculated.
The net current fed into the battery is determined by determining the charging current and the current consumption of the electronic device during the charging process (it should be noted that the device itself will normally consume some amount of current during the charging process), and calculating the net current fed into the battery therefrom. The charging current and the current consumption of the device can be determined, for example, by measurement of these values during the charging process, by suitable estimation procedures, by accumulation of the current fed into the battery or by any other appropriate technique. As one example, the charging current and the current consumption of the device can be measured, and the net current fed into the battery calculated therefrom. The net current fed into the battery, multiplied by the duration of the charging cycle, will represent a value of the total charge accepted by the battery during the charging process.
According to one presently preferred embodiment of the invention, the battery is completely discharged before the charging process is begun; and the charging step comprises fully charging the battery. In such circumstances, the total amount of charge accepted by the battery equals a value indicative of the total capacity of the battery. This value is then used to update the total battery capacity parameter provided by the cell vendor or previously calculated to permit remaining battery capacity to be more accurately calculated.
According to a further embodiment of the invention, it is not necessary to completely discharge the battery before the charging process is begun. Instead, a value representing the remaining capacity of the battery before it is charged is provided, and this value is added to the total amount of charge accepted by the battery during the charging process to provide the total battery capacity value.
Preferably, the step of providing a value representing the remaining battery capacity is made utilizing a xe2x80x9cremaining battery capacity estimationxe2x80x9d procedure according to yet a further embodiment of the present invention. In accordance with this procedure, the battery comprises at least one battery cell; and the voltage across the at least one cell is first determined, and this value is then used to determine the value representing the remaining battery capacity. The step of determining the voltage across the cell preferably comprises measuring the voltage across the battery terminals (i.e., the battery voltage) when battery current is sufficiently low, e.g., less than about 10 mA, such that the battery voltage and the cell voltage will be substantially equal, and after a period of time sufficient to stabilize any chemical reactions taking place in the at least one cell. The determined remaining battery capacity value is stored, and after the battery has been fully charged by the charging process, this stored value is added to the total amount of charge accepted by the battery during the charging process to provide the total battery capacity value.
In general, by providing a reliable value for the remaining capacity of the battery, complete discharge of the battery before charging is unnecessary. This can both simplify and speed up the overall calibration procedure somewhat.
The calibration procedure according to the present invention generally requires that the charging process be completed, i.e., that the battery be completely charged, so that an accurate total battery capacity value can be obtained. The present invention recognizes, however, that many users of portable electronic devices do not always permit the charging process to be completed for one reason or another. For example, when the battery is used in a mobile phone, the charging process might be interrupted to make a call. Also, high or low battery temperatures or other factors can also interfere with the charging process so as to prevent the battery from being fully charged.
According to a further embodiment of the invention, accordingly, if the charging process is interrupted or otherwise interfered with such that a complete charging of the battery can not be properly accomplished, an alternative calibration procedure is performed to ensure that the battery is calibrated in any event. For example, a calibration procedure based on the ageing of the battery can be carried out to provide an adjusted total battery capacity value. This will, at least, provide some degree of calibration when a full charging cycle is not carried out. Alternatively, and in accordance with further embodiments of the invention, a dynamic total battery capacity updating procedure can be performed during normal use of the device to permit adjustment of the total battery capacity value. These total battery capacity updating procedures can be used in conjunction with the normal calibration procedure during charging of the battery as described above, or separately as independent calibration procedures.
When a calibration procedure according to the present invention indicates that the total capacity value of the battery should be adjusted, it is preferred that the adjustment, whether it be an increase or a decrease in the value, be by no more than some maximum amount pursuant to any one calibration even if the results of the calibration indicate that a greater adjustment is needed. Such maximum amount can, for example, be a percentage of the previously stored total battery capacity value (e.g., 10%), a percentage of the amount of the adjustment indicated as being needed (e.g., 25-50%), or some other selected amount. By limiting the amount of the adjustment as a result of any one calibration, if an erroneous calibration does occur, it will have only a limited impact on the performance of the fuel gauging algorithm used by the device; and, in addition, it will prevent extreme adjustments to the total battery capacity value that might be noticeable to the user.
The present invention generally provides a calibration method and apparatus that can make fuel gauging more accurate than in many prior techniques, and that can more reliably provide the user of an electronic device with better information regarding the remaining operational time of the device. One reason for the increased accuracy is that errors that do occur during a calibration procedure tend to be cumulative in nature. Because, in the present invention, calibration can usually be done in less time than by prior methods, fewer errors are likely to occur such that the accumulated errors are also likely to be less.
In general, the present invention also provides a calibration procedure that is fully automatic and does not require any interaction from the user, other than the user initiating the normal process for charging the battery. The procedure is also fully adaptive inasmuch as a calibration will be performed at all available opportunities while, at the same time, a calibration will generally not be performed when there is a substantial risk of incorrect measurements. An important feature of the present invention is that it recognizes that different users of electronic devices have differing habits with respect to their use and handling of their devices; and endeavors to ensure that some type of calibration is periodically carried out irrespective of the user""s habits. The procedure of the invention also provides a convenient means to monitor the degradation of the battery due to ageing and/or other factors.
The above and further advantages, objects and features of the present invention will become readily apparent hereinafter in conjunction with the following detailed description of presently preferred embodiments thereof.